1. Field of the Invention
The present invention relates to a high frequency impedance measuring apparatus applicable to the impedance measurement of bodies of all kinds and particularly well adapted to the measurement of impedance in the hyperthermia field.
2. Description of the Prior Art
Impedance measurements have a fundamental interest in the field of hyperthermia where the problem consists in dissipating in the human body, or in localized regions thereof, an electromagnetic power delivered by a generator and applied to the region to be treated by means of application electrodes, this electromagnetic power being used for raising the temperature of the tissues.
The part thus treated forms, with respect to the application electrodes, an unknown impedance load which plays a very direct part in the transfer of power from the generator to the load; the frequency of the electromagnetic waves generally used in hyperethermia being between about 1 MHz and a few tens of MHz, remaining generally less than 50 MHz.
Up to now, the measurement of the real and imaginary parts of the impedance of the zone to be treated by hyperthermia uses means derived from low frequency impedance measurement means; these means being such for example as impedance bridge circuits used with a zero method or a substitution method. But the use of these means at relatively high frequencies means that precautions must be taken which are insufficient, such as: shielding the elements which are exposed to high frequency radiation, reducing as much as possible the parasite inductances and judicious interpretation of the results, taking into account all the disturbances which it is not possible to eliminate.
This leads to numerous and time-consuming manipulations which may prove injurious for the patient, the interpretation of the results being itself very difficult. Since hyperthermia treatment may require the consecutive treatment of several zones, which zones may have different impedances, the speed and accuracy with which these impedances are determined represent conditions of prime importance which are very imperfectly fulfilled at the present time.
At much higher frequencies such as microwaves frequencies where the wavelength is much smaller, the unknown impedance Z loading the end of a line may be determined after studying the intensity distribution of the electric field at different points of the line; the mismatching between the characteristic impedance Z of the line and the load impedance Z generating a standing waves system.
Measurement of the standing waves allows a coefficient of reflection .rho.=Rej.phi. to be derived, R being the modulus and .phi. the argument of this coefficient of reflection; since this latter is related to the impendence Z by the relationship EQU .rho.=(Z-Z.sub.c)/(Z+Z.sub.c),
the unknown load impedance Z can be determined.
To find the coefficient of reflecting .rho., a very well known method consists in moving a measuring probe along the line for measuring the field intensity at different points of this line and for finding the amplitude of the field maxima and minima. The amplitude ratio of the maxima to that of the minima is called standing wave ratio, whence is derived the modulus of the coefficient .rho.; the argument of this latter being derived from a measurement of the distance between the end of the line and the first field maximum.
The preceding method has however the drawback of involving a succession of time consuming and delicate manipulations and, considering the relatively large wavelengths used in hyperthermia, its use in his field will require an apparatus requiring considerable space.
To overcome these drawbacks, apparatus may be used derived from those described in these U.S. Pat. Nos. 3,986,113 and 4,506,209, and in the article by WOOD "Measurement of scattering parameters at 35 GMz using amplitude modulated homodyne detection" published in IEEE PROC vol 129, Pt H, No. 6, December 1982, pages 363-366.
These apparatus have however the drawback that they must be used with a matched generator of fixed power, which excludes the use thereof in fields such as hyperthermia where the apparatus operates with variable power (from 1 up to about 1000 watts). Furthermore, the mixers which they comprise require practically constant power levels a requirement which can only be satisfied in the laboratory and not in a hospital environment where the apparatus must be handled by operators generally having no qualifications in electronics.
The present invention provides then a measuring apparatus of the above mentioned type, which which may be used with an unmmatched generator delivering variable powers, requiring the simplest adjustments possible and being adaptable automatically to a large number of cases of use, over a large frequency range, without requiring a highly qualified operator, for hyperthermia, biomedical or any other type of application, particularly industrial.